perf: Optimize round scalar performance

datafusion/functions/Cargo.toml

@@ -320,3 +320,8 @@ required-features = ["math_expressions"]
 harness = false
 name = "floor_ceil"
 required-features = ["math_expressions"]
+
+[[bench]]
+harness = false
+name = "round"
+required-features = ["math_expressions"]

datafusion/functions/benches/round.rs

@@ -0,0 +1,154 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+extern crate criterion;
+
+use arrow::datatypes::{DataType, Field, Float32Type, Float64Type};
+use arrow::util::bench_util::create_primitive_array;
+use criterion::{Criterion, SamplingMode, criterion_group, criterion_main};
+use datafusion_common::ScalarValue;
+use datafusion_common::config::ConfigOptions;
+use datafusion_expr::{ColumnarValue, ScalarFunctionArgs};
+use datafusion_functions::math::round;
+use std::hint::black_box;
+use std::sync::Arc;
+use std::time::Duration;
+
+fn criterion_benchmark(c: &mut Criterion) {
+    let round_fn = round();
+    let config_options = Arc::new(ConfigOptions::default());
+
+    for size in [1024, 4096, 8192] {
+        let mut group = c.benchmark_group(format!("round size={size}"));
+        group.sampling_mode(SamplingMode::Flat);
+        group.sample_size(10);
+        group.measurement_time(Duration::from_secs(10));
+
+        // Float64 array benchmark
+        let f64_array = Arc::new(create_primitive_array::<Float64Type>(size, 0.1));
+        let batch_len = f64_array.len();
+        let f64_args = vec![
+            ColumnarValue::Array(f64_array),
+            ColumnarValue::Scalar(ScalarValue::Int32(Some(2))),
+        ];
+
+        group.bench_function("round_f64_array", |b| {
+            b.iter(|| {
+                let args_cloned = f64_args.clone();
+                black_box(
+                    round_fn
+                        .invoke_with_args(ScalarFunctionArgs {
+                            args: args_cloned,
+                            arg_fields: vec![
+                                Field::new("a", DataType::Float64, true).into(),
+                                Field::new("b", DataType::Int32, false).into(),
+                            ],
+                            number_rows: batch_len,
+                            return_field: Field::new("f", DataType::Float64, true).into(),
+                            config_options: Arc::clone(&config_options),
+                        })
+                        .unwrap(),
+                )
+            })
+        });
+
+        // Float32 array benchmark
+        let f32_array = Arc::new(create_primitive_array::<Float32Type>(size, 0.1));
+        let f32_args = vec![
+            ColumnarValue::Array(f32_array),
+            ColumnarValue::Scalar(ScalarValue::Int32(Some(2))),
+        ];
+
+        group.bench_function("round_f32_array", |b| {
+            b.iter(|| {
+                let args_cloned = f32_args.clone();
+                black_box(
+                    round_fn
+                        .invoke_with_args(ScalarFunctionArgs {
+                            args: args_cloned,
+                            arg_fields: vec![
+                                Field::new("a", DataType::Float32, true).into(),
+                                Field::new("b", DataType::Int32, false).into(),
+                            ],
+                            number_rows: batch_len,
+                            return_field: Field::new("f", DataType::Float32, true).into(),
+                            config_options: Arc::clone(&config_options),
+                        })
+                        .unwrap(),
+                )
+            })
+        });
+
+        // Scalar benchmark (the optimization we added)
+        let scalar_f64_args = vec![
+            ColumnarValue::Scalar(ScalarValue::Float64(Some(std::f64::consts::PI))),
+            ColumnarValue::Scalar(ScalarValue::Int32(Some(2))),
+        ];
+
+        group.bench_function("round_f64_scalar", |b| {
+            b.iter(|| {
+                let args_cloned = scalar_f64_args.clone();
+                black_box(
+                    round_fn
+                        .invoke_with_args(ScalarFunctionArgs {
+                            args: args_cloned,
+                            arg_fields: vec![
+                                Field::new("a", DataType::Float64, false).into(),
+                                Field::new("b", DataType::Int32, false).into(),
+                            ],
+                            number_rows: 1,
+                            return_field: Field::new("f", DataType::Float64, false)
+                                .into(),
+                            config_options: Arc::clone(&config_options),
+                        })
+                        .unwrap(),
+                )
+            })
+        });
+
+        let scalar_f32_args = vec![
+            ColumnarValue::Scalar(ScalarValue::Float32(Some(std::f32::consts::PI))),
+            ColumnarValue::Scalar(ScalarValue::Int32(Some(2))),
+        ];
+
+        group.bench_function("round_f32_scalar", |b| {
+            b.iter(|| {
+                let args_cloned = scalar_f32_args.clone();
+                black_box(
+                    round_fn
+                        .invoke_with_args(ScalarFunctionArgs {
+                            args: args_cloned,
+                            arg_fields: vec![
+                                Field::new("a", DataType::Float32, false).into(),
+                                Field::new("b", DataType::Int32, false).into(),
+                            ],
+                            number_rows: 1,
+                            return_field: Field::new("f", DataType::Float32, false)
+                                .into(),
+                            config_options: Arc::clone(&config_options),
+                        })
+                        .unwrap(),
+                )
+            })
+        });
+
+        group.finish();
+    }
+}
+
+criterion_group!(benches, criterion_benchmark);
+criterion_main!(benches);

datafusion/functions/src/math/round.rs

@@ -141,6 +141,50 @@ impl ScalarUDFImpl for RoundFunc {
             &default_decimal_places
         };
 
+        // Scalar fast path for float types - avoid array conversion overhead
+        if let (ColumnarValue::Scalar(value_scalar), ColumnarValue::Scalar(dp_scalar)) =
+            (&args.args[0], decimal_places)
+        {
+            // Extract decimal places as i32
+            let dp = match dp_scalar {
+                ScalarValue::Int32(Some(dp)) => *dp,
+                ScalarValue::Int32(None) => {
+                    return Ok(ColumnarValue::Scalar(ScalarValue::Float64(None)));
+                }
+                _ => {
+                    // Fall through to array path for non-Int32 decimal places
+                    return round_columnar(
+                        &args.args[0],
+                        decimal_places,
+                        args.number_rows,
+                    );
+                }
+            };
+
+            match value_scalar {
+                ScalarValue::Float64(Some(v)) => {
+                    let factor = 10_f64.powi(dp);
+                    return Ok(ColumnarValue::Scalar(ScalarValue::Float64(Some(
+                        (v * factor).round() / factor,
+                    ))));
+                }
+                ScalarValue::Float64(None) => {
+                    return Ok(ColumnarValue::Scalar(ScalarValue::Float64(None)));
+                }
+                ScalarValue::Float32(Some(v)) => {
+                    let factor = 10_f32.powi(dp);
+                    return Ok(ColumnarValue::Scalar(ScalarValue::Float32(Some(
+                        (v * factor).round() / factor,
+                    ))));
+                }
+                ScalarValue::Float32(None) => {
+                    return Ok(ColumnarValue::Scalar(ScalarValue::Float32(None)));
+                }
+                // For decimals and other types: fall through to array path
+                _ => {}
+            }
+        }
+
         round_columnar(&args.args[0], decimal_places, args.number_rows)
     }